I have done a mistake presuming that a 7 PCI-E board would take 6 GPUs and got the MSI Z170A KRAIT GAMING 3X  along with G3930 CPU and 2X4GB HyperX DDR4.

Please advice what board should I get to still be able to use the RAM and CPU with 6 RX470s.

Why on earth someone would build a 7 slot PCI-E board which cannot run more than 3 at a time 

Thanks